The present invention relates to a positive displacement vacuum pump, more particularly to a screw rotor type wet vacuum pump which can draw in by itself a sealing water supplied from a suction side. The wet vacuum pump can prevent direct contact of a pump casing with rotors due to thermal expansion caused by heat generated during an adiabatic compression step of the pump. The adiabatic compression step reduces an energy for driving the rotors.
A screw rotor type vacuum pump has been used for many applications in various fields such as gas vacuuming, gas suction, cleaning, and pneumatic conveying of powder, particles, and viscous materials.
FIG. 8 is a general illustration showing a sludge stripping unit which is an application example of the vacuum pump. In the sludge stripping unit, a sludge collection hopper tank 1 receives an end of a sludge suction pipe 2. The suction pipe 2 has a flange 3 positioned outside the hopper tank 1. The flange 3 is connected to a hose 4 for drawing in the sludge. The hopper tank 1 has a top wall provided with a conduit 7 communicating with an inner pipe 6 of a separator 5. The separator 5 has an air duct 8 positioned at an upper portion thereof. The air duct 8 is connected to a suction inlet of a vacuum pump A. A discharge side portion of the vacuum pump A is connected to an exhaust pipe 10 via a silencer 9.
Operating the vacuum pump A reduces the inner pressure of the separator 5 and the hopper tank 1. A worker puts the leading end of the sludge suction pipe 2 on the sludge, so that an air is drawn in together with the sludge through the sludge suction pipe 2 into the hopper tank 1. The sludge hits a top wall of the hopper tank 1 to be splashed backward, which allows a primary separation of the sludge and the drawn-in air.
The sludge having a comparatively large specific gravity falls to accumulate on a bottom wall of the hopper tank 1, while the air passes through the conduit 7 to flow downward in the inner pipe 6 of the separator 5. Then, the air passes through a liquid filled in the separator 5, allowing a secondary separation of the sludge and the air.
That is, the sludge included in the air is captured by the liquid, and only the air flows upward through a space outside of the inner pipe 6 into the air duct 8.
The air that has flown into the air duct 8 is drawn into the vacuum pump A, and then the air is discharged from a discharge port of the vacuum pump A into the silencer 9. Finally, the air is discharged in the atmosphere from the silencer 9 through the exhaust pipe 10.
When the sludge is stripped by means of the vacuum pump as described above, small amounts of entrained matters such as dust and pebbles still remain in the air even after the secondary separation. There is a fear of a damage of a sealing portion of the vacuum pump which draws in the air.
The screw rotor type vacuum pump A has a construction as illustrated in a longitudinal sectional view of FIG. 6. The pump has a housing 11 consisting of a main housing 12 having an inner cylinder 12a, a gear housing 13 closing a right end portion of the inner cylinder 12a, and a side cap 14 closing a left end potion of the inner cylinder 12a. 
The main housing 12 is provided with a suction port 15 communicating with the inner cylinder 12a, and the side cap 14 is provided with a discharge port 16 communicating with the inner cylinder 12a. 
The housing 11 accommodates a pair of screw rotors 17 (one of which is illustrated in FIG. 6) each consisting of a screw portion 17a and a shaft portion 17b provided at each end of the screw portion 17a. The screw portions 17a are of a Quimby type. The screw portion 17a has a normal section envelop consisting of a circular arc and a quasi-Alchimedean spiral curve.
The shaft portion 17b is rotatively supported by a fixed bearing 18 provided in the side cap 14 and by an expansion side bearing 19 provided in the main housing 12.
By sealing lines provided by the engagement of the pair of the screw portions 17a of the screw rotors 17 and the inner cylinder 12a of the main housing 12, there is defined an enclosed chamber 20.
By means of a pair of gears 21 each secured on each shaft portion 17b, the pair of screw rotors 17 rotate in opposite directions at the same speed as each other. Thereby, a fluid is drawn in from the suction port 15 of the main housing 12 into the enclosed chamber 20, and then the fluid is discharged from the discharge port 16 when the enclosed chamber 20 has moved to communicate with the discharge port 16.
For reducing the driving force of the vacuum pump, there is provided a discharge outlet 24 (see FIG. 2) for adjusting the open degree of the discharge port 16 to compress the drawn-in fluid at a compression rate of about 1/1.6 before discharging it.
FIG. 7 is a graph showing a relationship between a pressure (P) and a volume (V). The pressure is indicated by a vertical coordinate, and the volume is indicated by a horizontal coordinate. A one-step Roots vacuum pump and a screw rotor type vacuum pump with no adiabatic compression step each have an energy shown by a square area defined by points A, B, C, and D. Meanwhile, a screw rotor type vacuum pump with an adiabatic compression step has an energy shown by a semi-square area defined by points A, B, E, and D, which saves an energy shown by a diagonally shaded area of xcex94E.
The vacuum pump with an adiabatic compression step has to be a wet type pump to prevent direct contact of the screw rotor 17 and the housing 11 due to thermal expansion by heat generated during the adiabatic compression. The wet type pump generally draws in a sealing water by a vacuum generated in a suction port of the pump.
However, the water drawn in from a suction port 15 flows along a screw channel in an axial direction of the pump, and the water under pressure hits a discharge side shaft sealing portion 22 (see FIG. 6) like a water jet.
As, illustrated in FIG. 9, the shaft sealing portion 22 receives an increased force by the water pressure exerted thereon. The increased force produces no adverse effect for a service life of the shaft sealing portion 22, when the water is clean and includes no entrained matter such as dust or pebbles.
Meanwhile, when the water includes an entrained matter such as dust or pebbles, the shaft sealing portion 22 will have a reduced service life. If the shaft sealing portion 22 suffers a damage, the sealing water leaks into the fixed bearing 18 adjacent to the shaft sealing portion 22, which causes a breakage of lubrication of a grease fed in the fixed bearing 18. In addition, the deposition of the entrained matter such as dust or pebbles on the fixed bearing 18 tends to cause a damage of the fixed bearing 18.
To prevent the damage of the fixed bearing 18, a method is proposed, in which a slinger (or flinger) 23 is mounted on the shaft portion 17b as illustrated in FIG. 10. The slinger 23 rotates together with the screw rotor 17 to throw away a water leaked from the shaft sealing portion 22 to discharge it outside of the housing 11. Nevertheless, the method has the disadvantages that the discharged water makes an area surrounding the pump dirty and that a shortage of the sealing water occurs when a circulation system is applied for the water to save it.
In the present invention, when the suction pressure of a fluid is between a normal atmospheric pressure and xe2x88x92380 Hg, the fluid pressure of the discharge side becomes higher than the normal atmospheric pressure because, the fluid is compressed at a compression rate of about xc2xd in the discharge side. Meanwhile, when the suction pressure of the fluid is lower than xe2x88x92380 mmHg, the fluid pressure of the discharge side becomes lower than the normal atmospheric pressure. This lower discharge side pressure provides no leak of the sealing water from the shaft sealing portion 22 but acts to draw in the shaft sealing portion 22. Moreover, the inventors of the present invention have found that no direct contact of the screw rotor 17 and the housing 11 occurs even without a sealing water when the suction pressure of the fluid is higher than xe2x88x92380 mmHg. Through the use of these effects, the present invention eliminates the disadvantages described in the background of the invention.
To eliminate the disadvantages, a screw rotor type wet vacuum pump according to the invention includes a housing having an inner cylinder accommodating a pair of screw rotors engaging with each other. The screw rotors are of a Quimby type. Each of the rotors has a screw profile having a circular arc and aquasi-Archimedean spiral curve. A discharge port is opened when the rotation of the screw rotors 17 reduces the volume of an enclosed chamber at a compression rate of about 1/1.6. The enclosed chamber is defined by the rotors and the housing to receive a fluid drawn in from a suction port of the housing.
A feed pipe for a sealing water is connected to the housing to communicate with the enclosed chamber which is defined between a position in which a helical seal line of the screw rotors isolates the enclosed chamber from the suction side 6f the pump and another position in which the enclosed chamber begins to open to the discharge port.
Alternatively, a feed pipe for the sealing water is connected to the suction port of the vacuum pump, and the feed pipe is provided with a valve which opens when the suction pressure for the sealing water becomes lower than xe2x88x92380 mmHg.